Substitutes for glass covering products have already been proposed. The Applicants' French patent number 2 461 061 describes products obtained by making a sheet on a flat web papermaking machine or on a round mold papermaking machine by means of an aqueous composition, by removing the water from the sheet formed on the web in this way, and by drying it. The aqueous composition comprises:
cellulose fibers; PA0 non-cellulose fibers; PA0 at least one thermoplastic powder; PA0 at least one binder; and PA0 optionally fillers. PA0 satisfactory RTD, preferably greater than 300 N/m, and better still 500 N/m or more; PA0 adequate thickness, greater than 350 micrometers (.mu.m) and preferably greater than 450 .mu.m or even 500 .mu.m; PA0 high bulk; and PA0 dimensional stability at high levels of humidity.
The flocculant may be a cationic agent, for example, which imparts positive charge to the cellulose fiber. According to the Applicants' European patent application number 79 400 405.1, the flocculating agent is added in two stages and improves retention of the fillers and of the thermoplastic powders.
The Applicants' French patent number 82 12319 describes a paper sheet whose dimensional stability is improved by adjusting the quantity of glass fibers. This publication proposes coating or impregnating the paper sheet obtained in this way with an aqueous composition containing, in particular, a plasticizer, thermoplastic powders, and an emulsifier. The coating is then subjected to heat treatment in order to obtain partial gelling of the thermoplastic powders. The user of a paper sheet impregnated in this way then deposits various compositions of thermoplastic material thereon in order to obtain a covering material, e.g. a floor covering or a wall covering.
Such paper sheets must have excellent physical characteristics.
The sheets must be able to withstand being passed through industrial ovens at high temperatures, in particular while the floor or wall coverings are being manufactured. It is therefore necessary for the paper sheets to withstand heat well.
For floor coverings it is also desirable to obtain covering backing sheets having high resistance to traction-delamination (RTD). If this parameter is not correct, the sheet may delaminate in its own thickness while being used.
Finally, for this same floor covering application, it is necessary to obtain products which are particularly stable, dimensionally.
For reasons of cost, it is desirable to obtain sheets which have as low a density as possible.
The invention seeks to solve these problems. The aim of the invention is thus to obtain a sheet by a papermaking process which is usable as a backing for manufacturing floor or wall coverings and which has the following physical properties:
Finally, for unavoidable manufacturing considerations, adequate stiffness must be combined with these properties together with adequate sheet strength when hot since the sheet is intended to be covered with a plastic material and then subjected to heat.
The degree of dimensional stability under high humidity which is required for obtaining a product which is stable after being laid, i.e. without rolling or buckling, depends on the thickness and/or on the weight of the sheet. Thus, for a thick product having a thickness of 500 .mu.m or greater, an elongation of 0.25% in the crosswise direction appears to be a limit which should not be exceeded.
The person skilled in the art is aware that these requirements are often mutually incompatible. Thus, the person skilled in the art expects that if the thickness of the sheet increases, then its resistance to traction delamination will decrease. It is easily understandable that thick paper board delaminates more easily than thin paper board. The term "paper board" is used to designate any sheet of paper having high weight, namely more than 200 grams per square meter (g/m.sup.2).